Fc gamma receptors mediate antibody dependent inflammatory response and cytotoxicity as well as certain autoimmune dysfunction. We have determined the crystal structure of a human immunoglobulin receptor, FcgRIIIb, to 1.8 ? resolution. The overall fold consists of two immunoglobulin-like domains with an acute interdomain hinge angle of approximately 50 degrees. Trp 113, wedged between the N-terminal D1 and the C-terminal D2 domains, appears to further restrict the hinge angle. The putative Fc binding region of the receptor carries a net positive charge complementary to the negative charged receptor binding regions on Fc. Two separate parallel dimers are observed in the crystal lattice offering intriguing models for receptor aggregation.